A case study on how astronomical cycles affect the organic carbon accumulation

Abstract

The paleoclimate change impacts the sedimentary environment and process, which in turn control the accumulation of organic carbon. Numerous studies have shown that the paleoclimate is controlled by astronomical cycles. Hence, understanding how these cycles impact the accumulation of organic carbon is a critical question that requires in-depth discussion. Previous studies have shown that Milankovitch cycle can be revealed from the sediments of the 7th Oil Member (Chang 7 Member for short) of the Triassic Yanchang Formation in the southern Ordos Basin, suggesting that the deposition was controlled by astronomically-forced climate changes. Building on previous research, this study collected natural gamma (GR) data of Chang 7 Member from the N36 Well to further analyze astronomical cycles, combined with X-ray fluorescence (XRF) analysis and total organic carbon (TOC) tests, to reconstruct the paleoenvironment and analyze the organic matter enrichment characteristics. The results of this contribution show that, paleoclimate, paleo-redox conditions, paleo-water level, paleo-productivity and sediment accumulation rate (SAR) collectively controlled the enrichment of organic matter. Notably, this study identified the presence of eccentricity, obliquity, precession, and the 1.2 Myr long obliquity cycle in the Chang 7 Member. These cycles controlled the paleoenvironmental changes at different timescales and influenced the enrichment of organic matter, which has implications for subsequent energy exploration.